The role of heredity in the presence of asthma, severity of the condition, and impact of 12 specific triggers of attacks was investigated. Health surveys containing questions about children's asthma characteristics were completed by 325 families with twin children across the United States. Data for 39 monozygotic twin pairs and 55 same sex dizygotic twin pairs who were between 2 and 20 years of age and had asthma present in at least one member of each pair were received and analysed. Results revealed higher concordance for the presence of asthma among monozygotic (58.97%; n=23) than dizygotic twins (23.64%; n=13). Further analyses were restricted to data from the concordant monozygotic and dizygotic twin pairs. Asthma severity (the product of attack frequency and intensity ratings) was significantly correlated for monozygotic pairs but not for dizygotic pairs, and this difference in monozygotic and dizygotic severity correlations was significant. Also, monozygotic twins showed significantly higher correlations than dizygotic twins for the impacts of two asthma triggers: respiratory infection and physical activity. These results indicate a role of heredity in the presence of asthma and suggest that genetic factors may also affect the severity of children's asthma condition and the impact of respiratory infection and physical exertion as asthma triggers. (Arch Dis Child 1995; 73: 112-116)